Although not limited to medical treatment, circular radiation fields are often used in stereotactic radiosurgery (SRS) systems, such as Cyberknife™ or X-knife™. The dimensions of circular radiation fields used in SRS systems are relatively small and the effects of a flattening filter are small. The Cyberknife™ SRS system does not use any flattening filter. In linear accelerators used for conventional radiation therapy field asymmetry is defined as the percentage difference of any pair of points situated symmetrically with respect to the central axis. However, for small circular radiation fields like those produced from the Cyberknife™, the field asymmetry is defined as the percentage difference of two halves of the integrated profile, FIG. 1, where,Asymmetry=2(A1−A2)/(A1+A2),  (Equation 1)
Prior to this invention, circular radiation beam field symmetry assessment was accomplished either by film analysis, or by analyzing the beam profile acquired by scanning the entire field with an ion chamber or solid state detector system. In the above two methods, the field symmetry is obtained by comparing the integration of data points comprising each half-beam profile. The acquisition process of these data points comparison methods is rather lengthy. The objective of this invention is to obtain the field symmetry of circular radiation beams by directly measuring the integration of the half-beam profile, and thus simplify the assessment and quality assurance (QA) process in devices such as SRS linacs (Ref. 1, 2).
This invention provides an efficient and more convenient method for monitoring the field symmetry of circular radiation beams by directly measuring the integration of the half-beam profile.